A fuel cell is a device which directly converts fuel energy into electricity. In the fuel cell, an anode gas passage and cathode gas passage are provided on both sides of an electrolyte membrane. When hydrogen is supplied to the anode gas passage and oxygen is supplied to the cathode gas passage, electrical energy can be extracted according to the following electrochemical reaction occurring on the surface of the electrolyte membrane.                Anode reaction: H2→2H++2e−        Cathode reaction: 2H++2e−+(½)O2→H2O        
As it is necessary to maintain optimal humidification of the electrolyte membrane in order to extract the performance of the electrolyte membrane and improve the power generation efficiency of the fuel cell, the gas supplied to the fuel cell is humidified. Pure water is used as the water for humidification. This is because if the water contains impurities, the impurities will adhere to the electrolyte membrane and the performance of a fuel cell will be reduced.